The invention relates to a zeolite catalyst composition and an improved method of making a zeolite catalyst composition which has improved properties when compared with certain other zeolite catalysts. The improved method is especially important because it provides a zeolite catalyst composition with reduced coke formation properties and greatly simplifies the preparation of certain metal-promoted zeolite catalysts.
It is known to catalytically crack gasoline boiling range hydrocarbons (in particular, non-aromatic gasoline boiling range hydrocarbons, more in particular, paraffins and olefins) to light olefins, also referred to as lower olefins (such as ethylene and propylene), and aromatic hydrocarbons (such as BTX, i.e., benzene, toluene, and xylenes, and also ethylbenzene) in the presence of catalysts which contain a zeolite (such as ZSM-5), as is described in an article by N.Y. Chen et al. in Industrial & Engineering Chemistry Process Design and Development, Volume 25, 1986, pages 151-155. The reaction product of this catalytic cracking process contains a multitude of hydrocarbons such as unconverted C.sub.5 + alkanes, lower alkanes (methane, ethane, propane), lower alkenes (ethylene and propylene), C.sub.6 -C.sub.8 aromatic hydrocarbons (benzene, toluene, xylene, and ethylbenzene), and C.sub.9 + aromatic hydrocarbons. Depending upon the relative market prices of the individual reaction products, it can be desirable to increase the yield of certain of the more valuable products relative to the others.
One concern with the use of zeolite catalysts in the conversion of hydrocarbons to aromatic hydrocarbons and light olefins is the excessive production of coke during the conversion reaction. The term "coke" refers to a semi-pure carbon generally deposited on the surface of a metal wall or a catalyst. Coke formed during the zeolite catalyzed aromatization of hydrocarbons tends to cause catalyst deactivation. It is desirable to improve processes for the aromatization of hydrocarbons, and the formation of light olefins from hydrocarbons, by minimizing the amount of coke formed during such processes. It is also desirable to have a zeolite catalyst that is useful in producing significant quantities of the aromatic and olefin conversion products.
Certain known methods of preparing zeolite catalysts often require the modification of a zeolite or zeolite material with an acid to remove components which hinder the reaction and/or promote coke formation. The elimination of the process step of acid-treating, or acid-leaching, the zeolite can be desirable provided that it does not negatively impact the catalytic performance of the modified zeolite. The elimination of the acid-treating step can be particularly desirable if it results in an improved catalyst. There are also economic and safety benefits from an elimination of a process step involving the use of a strong acid.
It is also known that a thermally-cracked hydrocarbon-containing fluid in the gasoline boiling range, especially coker naphtha, may be produced by a coking process such as delayed coking, fluid coking, or contact coking, all of which are known processes in the petroleum refining industry. Because the coking process(es) are well known to one skilled in the art, the description of such coking process(es) is omitted herein.
Coker naphtha, being produced by a coking process, has a low octane number, typically no higher than about 70, and is a volatile material which is highly olefinic and diolefinic. Coker naphtha also tends to form gums by polymerization of diolefins and other unsaturated species which are present in the coker naphtha. Although the content of unsaturated species is high, with bromine numbers (ASTM D1159) typically in the range of 50 to 80, there is no positive contribution to octane from the unsaturated species as they are low octane components. Before the coker naphtha can be used elsewhere in a refinery, the coker naphtha must be severely hydrotreated to remove the olefinic and diolefinic materials. Such treatment results in an even lower octane number. Thus, the coker naptha must be further processed (for example, by reforming) before it can be used as a fraction in the gasoline boiling range with a high octane number, i.e., before it can be used as a motor fuel.
It is therefore desirable to improve the processes for the upgrading of a catalytically-cracked or thermally-cracked hydrocarbon-containing fluid in the gasoline boiling range, such as catalytically-cracked gasoline or coker naphtha, to reduce the levels of, or preferably remove, the low value olefinic and diolefinic materials (such as C.sub.5 + olefins and diolefins) from such hydrocarbon-containing fluid to produce a product containing high value petrochemicals such as aromatics (such as BTX, i.e., benzene, toluene, and xylene) and light olefins (such as ethylene, propylene, and butylene). It is also desirable to have a zeolite catalyst composition that is useful in the upgrading of such hydrocarbon-containing fluid, such as catalytically-cracked gasoline or coker naphtha, in a single-step process.
It is also known that these hydrocarbon-containing fluids are often contaminated with large amounts of nitrogen compounds. The presence of these nitrogen compounds can cause a loss of zeolite catalyst activity and stability. It is therefore desirable to have a process that does not significantly decrease the activity and stability of a zeolite catalyst when such catalyst is used in the conversion of hydrocarbons, preferably during the upgrading of hydrocarbon-containing fluids such as coker naphtha.